For the most part, in such production processes the coating is applied to the strip as a liquid paint, in which the constituents, namely the polymeric materials, pigments and maybe inert fillers, are held in a solvent. The paint constituents are left as a film on the strip as a result of the solvent evaporating, either naturally or on being heated. If the film forming polymeric material is of the thermoplastic type, the removal of the solvent virtually completes the process. If the polymeric material is of the thermosetting type, a subsequent heating is required to effect the cross-linking which causes the film to harden.
In large scale plants, the storage handling and, in particular, the recovery, of the considerable volumes of environmentally hazardous solvents involved, requires care and the provision of expensive equipment, if the working environment is to be satisfactory.
In other situations, it is known to use the so called dry-powder coating process. Briefly stated, this process comprises spraying electrostatically charged, dry particles of a paint composition on to an oppositely charged article to be coated, and then heating the powder layer to form a film and, if needed, thermoset the film. That process eliminates the solvent, but is not suitable for continuous strip coating because of the severe limitations the application of the powder would impose on the strip speed. Furthermore, powder coatings are inherently thicker than those preferred on stock strip material.
Therefore, it is an object of the present invention to provide a coating process which is suitable for use in a high speed, continuous coating plant and which eliminates the need for solvent recovery.
A further disadvantage of conventional practice using liquid coating compositions, is the wastage, and more importantly, the time delays, associated with changing from one coating to another to suit requirements for differently coloured, or otherwise differently coated, products. At such a changeover it is necessary to drain and clean the pipe lines, pumps, applicator and all other equipment contacted by the coating material, to avoid contamination of the fresh coating with the old. This has led to long product runs, with consequent large inventories of finished stock and lack of flexibility in meeting orders for small quantities of seldom required product.
Ideally, production would be scheduled to meet each individual order as received, and another object of the present invention is to provide a continuous strip coating process enabling a changeover time which is small enough to permit that ideal situation to be largely attained.